Wolfram Research, Inc.
ReplaceAllUnheld[expr, rules] applies a rule or list of rules in an attempt to transform each subpart of expr that would be automatically evaluated.
ReplaceAll operates on all subparts of an expression; ReplaceAllUnheld operates only on those subparts that would normally be evaluated.
Example: Developer`ReplaceAllUnheld[If[a, a, a], a->b] .
See also: ReplaceAll, Hold, Verbatim.